1. Field of the Invention
This invention relates to an image pickup device and a video signal processing method that can suitably be used in a monitoring video camera. More particularly, the present invention relates to the technology for broadening the dynamic range of a video signal so that the picked up image of the object including dark areas and bright areas may be clearly viewed with a sufficient contrast (gradation) effect.
2. Description of the Related Art
Conventionally, the image pickup device that is used in a monitoring video camera typically comprises an image pickup element such as a CCD (charge-coupled device) and the video signal obtained from the image pickup element is subjected to a predetermined signal processing operation (KNEE, gamma, detail, white balance, etc.) to produce a final video signal to be used for producing an image of the object.
However, image pickup devices such as CCDs generally show a narrow dynamic range and hence it is difficult to pick up an image of an object having a large difference of luminance between the dark areas and the bright areas thereof. Techniques for broadening the dynamic range by adding or synthesizing two video signals obtained from an object in different imaging conditions and weighted appropriately in order to alleviate the above identified problem are known. An image pickup device disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-209492 (to be referred to as the first prior art document hereinafter) will be briefly described below as one of such known techniques.
According to the first prior art document, an image pickup device is adapted to obtain sequentially a first video signal for an image picked up by appropriately opening the diaphragm and a second video signal for an image picked up by narrowing the opening of the diaphragm. Then, the first video signal is compressed to a specified amplitude level. In the case where the compressed signal exceeds a threshold value that represents a predetermined brightness, the compressed first video signal is attenuated by using a predetermined coefficient while the second signal is amplified by using another predetermined coefficient and then the two signals are added to produce a video signal showing a broad dynamic range.
Other prior art techniques for broadening the dynamic range of a video signal are disclosed in Jpn. Pat. Appln. KOKAI Publication No. 10-233966 (to be referred to as the second prior art document hereinafter) and Jpn. Pat. Appln. KOKAI Publication No. 11-164195 (to be referred to as the third prior art document hereinafter).
An image pickup device according to the second prior art document is adapted to obtain sequentially two video signals with different extents of exposure to light by means of an image pickup element that is a CCD. Then, the video signal obtained with the smaller extent of exposure is corrected for gradation and the corrected signal is synthetically combined with the video signal that is obtained with the larger extent of exposure in order to provide the final video signal with a sufficient gradation effect for bright areas.
Now, an image pickup device according to the third prior art document is adapted to obtain sequentially two video signals respectively with a long exposure time and a short exposure time and hence with different brightness levels, which two video signals are then synthetically combined to produce a final video signal by using a composite function selected out of a predetermined number of composite functions depending on if the maximum luminance level exceeds a reference value or not.
However, with any of the image pickup devices according to the above cited first through third prior art documents, while high luminance areas provide a sufficient gradation effect, the obtained image can provide only a poor contrast effect when an object having a narrow dynamic range is picked up. This is because the video signal of an image picked up with a scarcely opened diaphragm (and hence with a small extent of exposure to light) does not provide a satisfactory level of luminance so that the video signal of a corresponding image picked up with a normally opened diaphragm (and hence with a relatively large extent of exposure to light) is compressed to below the predetermined reference level although no signal will be practically added thereto.
Additionally, when an image of an object showing a broad dynamic range is picked up, the parts of the signal obtained by addition show a fixed dynamic range to make the bright areas of the produced final image show only a poor contrast effect because the video signal of the image picked up with a normally opened diaphragm (and hence with a relatively large extent of exposure to light) is compressed to a fixed reference level.
Meanwhile, a system of obtaining two video signals sequentially under different exposure conditions provides an effect of broadening the dynamic range at low cost when it is applied to a conventional system for synthetically combining images obtained with different electronic exposure durations (a long exposure time of {fraction (1/60)} seconds and a short exposure time) by using a CCD and a signal processing IC. However, the image obtained by means of such a system provides a vertical resolution that is only a half of the vertical resolution of an ordinary image (obtained by an ordinary imaging operation without the synthesizing process) and the number of images picked up per unit time is halved (e.g., 60 images per second to 30 images per second). This means that a jerkily and strangely moving image showing a low vertical resolution may be obtained from an object that does not require a wide dynamic range.